Preparing Flet v0.28.3 release

[FeodorFitsner]

Summary by Sourcery

Prepare v0.28.3 release with support for multiple pubsub subscribers, mobile platform file picker fixes, routing/event bug resolutions, and dependency and changelog updates.

New Features:

• Allow multiple subscribers per session and per topic in the pubsub hub

Bug Fixes:

• Fix local image rendering on Android by correcting asset URI construction
• Resolve blank gray screen and missing fileName in FilePicker.save_file() on mobile platforms
• Restore correct view pop routing by matching views to route names

Enhancements:

• Remove redundant typing import fallback in Python file picker module

Build:

• Bump Python and Flutter package version to 0.28.3 and update Flutter file_picker dependency to ^10.1.9

CI:

• Update milestone number in the whats-new PowerShell script

Documentation:

• Add 0.28.3 release notes to SDK and package changelogs